"I'm a believer"

Room dimensions, furniture, occupancy of the room, wall hangings, open or closed door and the list goes on and on. All these with even the most minute of change are factors in what we hear.

Because most people never tried to acoustically control a room , passively and actively with mechanical devices as resonators. they cannot imagine the deep audible powerful impact of the presence of even a single straw of specific dimension and location in a room .. Period...

Remember that a speaker is a resonator among possible other resonators in the room ...their distribution matter ...

I too have heard speakers that alone double my investment in my whole system, and walked away thinking ‘what am I not getting’ for that kind of money. And others times I hear speakers that I would think of as small room system speakers and thought ‘why in the heck did I spend what I spent’ .

The reason why this is so, is because the acoustic control of the couple speakers/room/ears is the main factor in audio not the dimension of the speakers or his price tag or even measured specs ... Any speakers well designed of any price in an adapted room will sound at least reasonnably good if you give it to someone knowing basic acoustic ...

It will reach and pass the minimal acoustical satisfaction threshold ...This threshold exist not only subjectively but objectively ... As the diminishing returns threshold which is also a subjective/objective FACT determined by subjective history of the owner and objective qualities of the design and acoustic coupling ...

It's the ROOM!

In my experience, any speaker or other type of audio equipment will only truly shine in a properly treated room.

I've been to three audio shows in the past several years (Axpona, Tampa, PAF) and I have come to the conclusion that the concept of diminishing returns doesn't apply very well to speakers. Rather, there is just no solid correlation between price and performance. Like the OP I have heard megabuck speakers that didn't sound good and I've heard several moderately priced speakers that sounded wonderful. I don't think that I can attribute this to non-optimum amplification or source equipment because I trust the speaker manufacturer to know what components work best with their speakers. In many demonstrations the cables and power conditioning cost more than my entire system and I my system was better, at least to me.

To put it another way, I've heard six-figure speakers that sounded amazing and I've heard others that sounded mediocre. I'm a geezer - I've had this hobby for 50 years - and I've concluded that once you get into the audiophile range of gear (thousands of dollars per piece, not hundreds) the value proposition seems to break down. One would think that a million dollar system would be life changing but I have not found that to be the case. It's not that diminishing returns takes over, it's more that it's a crapshoot. My favorite speakers of all time are the MBL 101 E Mk II and while they are certainly expensive ($80K) they outperformed speakers that were multiples of their price.

We have to take into account that a manufacturer’s "statement piece" is just that. It is a statement of the capabilities of the manufacturer. This would suggest a "no compromise" approach to EVERY aspect of the speaker in the range of "world class" speakers.. This would mean that everything you see, touch, and HEAR would stand up to (or exceed) the best of the best. Including esthetics.

I would suggest that the "law of diminishing returns" starts at about $50. Part’s Express offers a pair of 4 1/2" 2-way bookshelf speakers for around that price. Okay, you double the price ($100) and get bass extension. So, are improvements to 2 of the 10 octaves worth TWICE the price. "Common sense" say’s "No!!" The emotional, audiophile side of us says "Yes!. So, we keep doubling the price until we arrive at, say, $60k. Is a $120k speaker twice as good? Just revert back to the $50-$100 upgrade example for your answer. The same rules apply.

So the "ultra top end" has a market for those whose life choices are "this AND that", unlike us morals who have to choose between "this OR this."

Hope those six figure speakers hang around for a while. Even if their "diminishing returns factor" don’t pass the "4th grade math" test, yet put a smile on the faces of their owners.

There is definitely a Rule of Diminishing return just as there is the Sunk Cost Fallacy. I suspect that the Venn diagram of those two circles would be close to one circle in some cases for certain audiophiles. For example, someone subconsciously suffering from the Sunk Cost Fallacy has spent years and enormous amounts of money in their pursuit of perfection and yet never attain it as perfection does not exist. So each subsequent purchase is rationalized as moving closer to audio perfection. Yet the reality is they would not / could not admit - even with hard facts -  that the recently purchased $100K speakers are marginally better or equal to the $10K speakers or even the $5K speakers because they suffer from the Sunk Cost Fallacy. The diminishing returns set in long ago for each dollar spent.

The Rule of Diminishing Return applies to many industries where the marginal performance gains do not justify the cost, yet at the high-end market of consumable goods, the real reason is justification for many that they can exhibit how they can "afford" to pay $150K for certain speakers, or $200K for a vintage Jaguar, or any luxury item for that matter. Some may actually know they're getting no real qualitative improvement for their investment because the point is to exhibit their wealth.